1. Field of the Invention
This invention relates to a method of producing a high purity naphthalene dicarboxylic acid.
2. Related Art Statement
Naphthalene dicarboxylic acid (hereinafter abbreviated as NDCA), particularly 2,6-naphthalene dicarboxylic acid (2,6-NDCA) is noticed as a starting material for dye and medicine and a starting material for polyethylene naphthalate resin (PEN) and liquid crystal polyester resin (LCP). In case of PEN using dimethyl 2,6-naphthalene dicarboxylate (2,6-NDCA dimethyl) as a starting material, since 2,6-NDCA dimethyl is easy in the purification and available in a high purity, the production has already been put into practical use. However, in case of LCP using 2,6-NDCA as a starting material, excellent properties are attained, but it is difficult to obtain a high purity 2,6-NDCA, so that the production is not yet put into practical use.
The reason why the production of high purity 2,6-NDCA is difficult is due to the fact that the purification is very difficult as mentioned in the features 1)-3):
1) There is substantially no solvent sufficiently dissolving 2,6-NDCA, so that the recrystallization is impossible.
2) The vapor pressure of 2,6-NDCA is very low, so that the purification through distillation or sublimation is impossible.
3) Since 2,6-NDCA does not melt (which has no melting point and decomposes by heating), the crystallization is impossible.
Up to the present, many methods of purifying 2,6-NDCA have been proposed, but they are not for direct purification. For example, there have been known a method wherein 2,6-NDCA is converted into an acid chloride and then purified (Japanese Patent laid open No. 62-169747), a method wherein 2,6-NDCA is converted into an alkali salt and then purified (Japanese Patent Application Publications No. 45-7738, No. 45-13096, No. 52-20993, No. 52-20994, No. 56-3858 and No. 57-36901), a method wherein 2,6-NDCA is converted into an amine salt and then purified (Japanese Patent Application Publications No. 56-48498 and No. 57-14331), a method wherein 2,6-NDCA is converted into an ammonium salt and then purified (Japanese Patent laid open No. 51-52163) and the like.
In the method of purifying 2,6-NDCA after the conversion into acid chloride, however, it is required to use an expensive halogenating agent such as thionyl chloride. Furthermore, this method can not be said to be an industrially suitable one considering the treatment for chloride by-produced in the turning of acid chloride to carboxylic acid and the material of the equipment durable to corrosion. On the other hand, in the methods of purifying after the conversion into various salts, it is difficult to remove position isomers and monocarboxylic acid included as impurities in 2,6-NDCA, and a high purity usable as a starting material for LCP can not be obtained.
A simple method of producing a high purity 2,6-NDCA usable as a starting material for LCP is the production from 2,6-NDCA dialkylester. Since 2,6-NDCA dialkylester, for example, 2,6-NDCA dimethyl can be purified by distillation or recrystallization, crude 2,6-NDCA is converted into dialkylester with an alcohol or the like and purified to obtain a high purity 2,6-NDCA dialkylester, from which a high purity 2,6-NDCA can be obtained.
However, when 2,6-NDCA dialkylester is converted into 2,6-NDCA through hydrolysis with a strong alkali, if the strong alkali is sodium hydroxide, the resulting 2,6-NDCA disodium salt is hardly soluble in water, so that the reaction solution changes to a gel as the hydrolysis reaction proceeds and finally renders into a solid soap to stop the reaction. Even when using potassium hydroxide as a strong alkali, as the reaction proceeds, the reaction solution changes to a slurry having a high viscosity, so that the reaction should be carried out with a fairly diluted solution for preventing such a change. Furthermore, the resulting 2,6-NDCA consumes the alkali to produce a salt, so that it is necessary to use an alkali in an amount of not less than equivalent of resulting carboxyl group. Moreover, the particle size of 2,6-NDCA in the slurry produced when the resulting 2,6-NDCA dialkali salt is neutralized with an acid is very small, so that the slurry has a high viscosity and a high water content. As a result, the treatment for solid-liquid separation is charged with difficulties and also the washing becomes difficult, so that there are caused problems that the alkali metal salt by-produced in the neutralization incorporates into final 2,6-NDCA and a great energy is taken for the drying after the washing.
On the other hand, the use of the acid catalyst can substantially avoid the aforementioned drawbacks caused in the hydrolysis with the alkali catalyst, but the reaction is very slow and can not put into practical use. For instance, even when 2,6-NDCA is boiled in an aqueous solution of strong acid, hydrolysis reaction does not proceed at all.